A multiblade centrifugal fan in which an impeller having a plurality of blades is disposed in such a manner as to be rotatable via a motor within a scroll-shaped casing with a tongue section thereof serving as a base point is widely used as an air-blowing fan in refrigerators, air conditioners, or ventilators (which will simply be referred to as “air conditioners” hereinafter). In such a multiblade centrifugal fan, air taken in through an inlet, provided at an upper end surface of the scroll casing, in the axial direction is deflected in the centrifugal direction (i.e., the radial direction) from the inner periphery toward the outer periphery as it passes between the blades of the impeller. The air is pressure-fed so as to be blown from the impeller to an air flow path within the scroll casing. Subsequently, the air is delivered in the rotational direction along the inner peripheral surface of the scroll casing so as to be blown outside via an outlet.
In the aforementioned multiblade centrifugal fan, in order to recover the dynamic pressure of the air flowing through the scroll casing in the rotational direction of the impeller, the scroll casing has the shape of a scroll with an outer diameter that gradually increases in the rotational direction. In recent years, in order to make the casing as compact as possible, the casing is extended in the rotation-axis direction such that the cross section of the flow path gradually increases in the rotational direction. In such a multiblade centrifugal fan, the air blown into the scroll casing from the entire perimeter of the impeller is not completely deflected within the impeller, but is blown downward at an angle lopsidedly toward a lower end surface of the scroll casing, and then flows in the rotational direction. In this case, the air is blown toward the outlet while generating rotational flows (vortex flows) proceeding toward the inner periphery at the upper and lower sides of the flow path when viewed in cross section.
In particular, with regard to the rotational flow generated at the lower end surface opposite the upper end surface provided with the inlet in the scroll casing, when the flow proceeding toward the inner periphery above the lower end surface strikes an inner-peripheral side surface of an extension section extended in the rotation-axis direction and then proceeds toward the upper end surface (i.e., upward), this flow interferes with the airflow from the impeller, causing disturbance in the flow, which results in problems such as increased aerodynamic noise and reduced air-blowing efficiency. Patent Literature 1 proposes an example in which the inner-peripheral side surface of the extension section extended in the rotation-axis direction of the casing is formed as an inclined surface, and a plurality of ribs extended in the rotational direction are provided on this inclined surface. By means of the ribs, a secondary flow proceeding toward the impeller is suppressed, thereby reducing noise.